Plate freezer evaporator with carbon dioxide refrigerant

ABSTRACT

An evaporator for use in a plate freezer utilizing carbon dioxide as the refrigerant has a duct with an elliptical shaped cross section. The duct allows a refrigerant, such as carbon dioxide, to pass through the evaporator to create a low-temperature freezer space.

FIELD OF THE INVENTION

[0001] The present invention generally relates to plate freezers, andmore particularly, the invention relates to an improved design of aplate freezer evaporator for accommodating increased refrigerantpressures associated with the use of carbon dioxide as a refrigerant.

BACKGROUND OF THE INVENTION

[0002] Plate freezers are generally known in the art. They are highefficiency freezers used in a variety of applications, usually in thefood processing industry. Typically, one or more spaced-apartheat-exchanger plates are located in the freezer compartment. Arefrigerant passes through each of the plates to lower the temperatureof the plates and the freezer compartment. The items to be frozen arethen placed on the refrigerated plates. The high efficiency of platefreezers allow for a reduction in the size of the freezer compartmentand for a more rapid freezing than in typical cold air freezers wherethe cold air is simply blown over the items until frozen.

[0003] One commonly used thermodynamic cycle for plate freezerapplications is known as a vapor-compression refrigeration cycle. Inthis cycle, a superheated vapor refrigerant is compressed in acompressor, causing an increase in temperature. The hot, high pressurerefrigerant is then circulated through a heat exchanger, called acondenser, where it is cooled by heat transfer to the surroundingenvironment. As a result of the heat transfer to the environment, therefrigerant condenses from a gas to a liquid. After leaving thecondenser, the refrigerant passes through a throttling device where thepressure and temperature both are reduced. Upon exiting the throttlingdevice, the refrigerant enters a second heat exchanger, called anevaporator, located in the freezer space. In plate freezers, theevaporator includes a plate surface upon which the items to be frozenare placed. Heat transfer in the evaporator causes the refrigerant tochange from a liquid phase to a saturated mixture of liquid and vapor.The vapor leaving the evaporator is then drawn back into the compressor,and the cycle is repeated.

[0004] In recent years, concern for the environment has brought about aphase-out of many refrigerants traditionally used in vapor-compressionrefrigeration systems. This phase-out of traditional refrigerants, suchas chlorofluorocarbons (“CFCs”), occurred since their release into theenvironment depleted the ozone layer in the stratosphere. The use andemission of these refrigerants are now regulated through the terms ofthe 1987 Montreal Protocol on Substances that Deplete the Ozone Layer.The 1987 Montreal Protocol places stringent limitations on the use ofCFC refrigerants. As such, there has been an immediate shift away fromCFCs toward refrigerants that are more environmentally friendly.

[0005] The effort to find thermodynamically suitable refrigerants thatdo not adversely affect the ozone layer has led to the use of ammonia(NH₃) and carbon dioxide (CO₂) as refrigerants. These refrigerants havevirtually no ozone depletion potential. Despite its environmental appealas a refrigerant, ammonia (NH₃) has a pungent, suffocating odor and istoxic and flammable under certain conditions.

[0006] The use of carbon dioxide as a refrigerant also has certaindrawbacks. One difficulty associated with the use of carbon dioxide as arefrigerant is the high working pressure of the carbon dioxide. Intypical plate freezer applications, the working pressure of the carbondioxide ranges from approximately 100 psig (690 kPa) to about 300 psig(2070 kPa). The required refrigerant pressure associated with the use ofcarbon dioxide can create unacceptable stress levels in the componentsof a plate freezer.

[0007] An additional difficulty associated with the use of carbondioxide is the pressure increase associated with a “shut-down” of thefreezer. Freezer shut-down can occur through an interruption in thesource of electrical power as well as the intentional shut-down of thefreezer for defrosting or servicing. In the event that a freezershut-down causes the carbon dioxide to reach room temperature, therefrigerant can reach pressures in excess of 1000 psig (6900 kPa). Thepressure increase has been addressed through the use of a system ofrelief valves, such as those shown generally in U.S. Pat. Nos. 4,986,086and 5,042,262. However, the use of a relief valve system requiresrefilling of the refrigeration system with refrigerant lost through therelief valve before the freezer can be restarted.

[0008] Accordingly, it would be desirable to have an evaporator for usein a vapor-compression refrigeration cycle which uses carbon dioxide asa refrigerant. Furthermore, it would be desirable to accommodateincreased working pressure when using a carbon dioxide refrigerant, aswell as the increased in carbon dioxide pressure during shut-downwithout loss of refrigerant while maintaining stress levels in theevaporator substantially below the yield strength of the material fromwhich the evaporator is constructed.

SUMMARY OF THE INVENTION

[0009] Accordingly, it is a general object of the invention to overcomethe deficiencies of the prior art.

[0010] It is a more specific object of the present invention to providean improved evaporator for use in a vapor-compression refrigerationcycle.

[0011] It is a further object of the present invention to provide anevaporator for use in a plate freezer in which carbon dioxide is used asthe refrigerant.

[0012] It is another object of the present invention to address highrefrigerant pressures associated with freezer shut-down without loss ofrefrigerant.

[0013] The present invention provides these and other additional objectswith a plate freezer evaporator which uses carbon dioxide as arefrigerant. The evaporator is adapted to accommodate refrigerantpressures associated with ordinary freezer operation as well as theelevated refrigerant pressures, such as those encountered during freezershut-down. The evaporator includes a longitudinally extending plate bodyhaving a first generally planar heat transfer surface, a secondgenerally planar heat transfer surface spaced apart from the first heattransfer surface, to define a plate body solid volume. The evaporatoralso includes at least one longitudinally extending duct passing throughthe plate body solid volume to channel a refrigerant (such as carbondioxide) maintained at a relatively high pressure. The duct has anelliptical cross-section designed to maintain a stress level in theplate body at a level substantially below the yield strength of thematerial from which the plate body is constructed. By forming the ductin this fashion, the evaporator accommodates refrigerant pressuresassociated with ordinary freezer operation as well as the elevatedrefrigerant pressures, such as those encountered during freezershut-down.

[0014] Other objects and advantages will become apparent upon readingthe following detailed description and upon reference to the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a perspective view of two plate freezer shelves whichare comprised of a plurality of individual freezer plates;

[0016]FIG. 2 is a partially-exploded perspective view of one of theplate freezer shelves shown in FIG. 1;

[0017]FIG. 3 is a partial cut-away enlargement of a portion of FIG. 2illustrating a refrigerant flow path;

[0018]FIG. 4 is a cross-sectional view of a freezer plate along line 4-4in FIG. 2;

[0019]FIG. 5 is a cross-sectional view of a header along line 5-5 inFIG. 2;

[0020]FIG. 6 is a cross-sectional view of a freezer plate along line 4-4in FIG. 2 showing the displacement of the freezer plate when theinternal pressure in the plate is approximately 1400 psig (9660 kPa);

[0021]FIG. 7 is a cross-sectional view of a header along line 5-5 inFIG. 2 showing the displacement of the header when the internal pressurein the header is approximately 1400 psig (9660 kPa); and

[0022]FIG. 8 is a diagrammatic illustration of a mechanicalrefrigeration system for use in conjunction with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Generally, the present invention relates to a plate freezerevaporator, having a duct with an elliptical cross section, locatedbetween two heat exchanger surfaces which define a generally solid platebody with the elliptical duct present in each plate body. The shape ofthe duct allows the evaporator to accommodate carbon dioxide pressuresassociated with ordinary freezer operation as well as the elevatedrefrigerant pressures encountered during freezer shut-down. Althoughdescribed in the context of a plate freezer utilizing carbon dioxide asa refrigerant, it should be understood that the invention is not limitedto such applications. The invention may be used in other refrigerationor heat transfer applications which utilize a variety of working fluidsin lieu of or in addition to carbon dioxide.

[0024]FIG. 1 is a perspective view of two freezer shelves and spacedapart from one another. Each freezer shelf is adapted to receive itemsto be frozen between the adjacent shelves. Each of the shelves comprisesof a plurality of generally rectangular freezer plates 20. Each freezerplate 20 has a length that is substantially greater than the width.Adjoining freezer plates are placed adjacent to one another along theirrespective lengths and attached to one another in this side-by-sideorientation to create a plate freezer shelf as shown in FIGS. 1 and 2.The resulting plate freezer shelf is then connected to an inlet header32 and an outlet header 34.

[0025] During ordinary operation of a plate freezer, carbon dioxide ispumped into the freezer plates 20 from the liquid manifold 28 throughone of the several inlet hoses 30. The carbon dioxide then flows througha serpentine circulation duct (reference numeral 50 in FIGS. 2, 3, 4 and6) located in each of the freezer plates 20. After the carbon dioxidehas passed through the entire length of the circulation duct located ineach freezer plate 20, it flows into the outlet headers 34.

[0026] Due to heat transfer to the carbon dioxide from the items locatedon the plate freezer to be frozen, the carbon dioxide exits the outletheaders 34 as a saturated mixture of liquid and vapor. This saturatedmixture enters the collecting manifold 38 through one of the outlethoses 36 leading from the outlet headers 34 to the collecting manifold38. The saturated mixture of liquid and vapor carbon dioxide then exitsthe collecting manifold 38 towards the primary receiver 24 shown by flowarrow 26 c in FIG. 8.

[0027] A plurality of freezer plates 20 are attached to one another andare positioned in spaced-apart relation to create a plurality of platefreezer shelves. The plurality of freezer plates 20 which comprise aplate freezer shelf are located within the sealed freezer space. Theitems to be frozen are placed on a freezer plate shelf. Although FIG. 1illustrates the plate freezer shelf as being comprised of thirteenindividual freezer plates 20, any number of freezer plates 20 can beused to create a freezer plate shelf of the desired size. Additionally,a plurality of freezer plate shelves can be located in a particularfreezer space to maximize the number of items which a given freezervolume can accommodate for freezing.

[0028]FIG. 2 is a partially-exploded perspective view of the freezerplates 20 assembled to form a single plate freezer shelf. FIG. 2illustrates the circulation of the carbon dioxide from the inlet header32, through the freezer plates 20 and into the outlet header 34.

[0029] In FIG. 2, the carbon dioxide enters the inlet header 32 throughthe inlet hose 30. Located in the inlet header 32 is a plurality ofinlet header apertures 48. Each inlet header aperture 48 is in fluidcommunication with a serpentine circulation duct 50 located within eachfreezer plate 20. The fluid communication between the apertures 48 andthe circulation duct 50 allows carbon dioxide to flow from the inletheader 32 into the freezer plate 20. Shown in FIG. 2, each individualfreezer plate 20 has a single serpentine circulation duct 50 with eachduct having a single inlet 52 and a single outlet 56 with each inlet 52in fluid communication with an inlet header aperture 48. Theserpentine-shaped circulation duct 50 allows carbon dioxide to flowthrough the freezer plate 20 while having sufficient residence timewithin the freezer plate to allow heat transfer to occur and cool thefreezer plate 20.

[0030] Although FIG. 2 illustrates each freezer plate 20 as having asingle serpentine circulation duct 50 with a single inlet 52 and outlet56, it will be appreciated by those of skill in the relevant art thateach freezer plate 20 may have multiple serpentine ducts 50 and may alsohave multiple inlets and outlets and still be within the scope of thepresent invention.

[0031] Upon flowing through the entire length of the circulation duct50, the carbon dioxide exits the freezer plate 20 and enters the outletheader 34. The outlet header 34 contains a plurality of outlet headerapertures 54 which are in fluid communication with a circulation ductoutlet 56 such that the carbon dioxide can flow from the circulationduct 50 into the outlet header 34 through the outlet header apertures54. The carbon dioxide then flows the length of the outlet header 34 andexits through the outlet hose 36 into the collecting manifold 38 shownin FIGS. 1 and 8.

[0032]FIG. 3 is a partial cut-away enlargement of a portion of FIG. 2illustrating a flow path of carbon dioxide from the inlet hose 30,through the inlet header 32 and into the freezer plate 20. Inparticular, the carbon dioxide enters the inlet header 32 through theinlet hose 30 from the liquid manifold 28 (shown in FIGS. 1 and 8). Thecarbon dioxide enters the first channel 58 of the inlet header 32 andflows through a series of apertures 60 in the web 62 which separates thefirst channel 58 from the second channel 64 of the inlet header 32. Uponentering the second channel 64 of the inlet header 32, the carbondioxide flows through the inlet header aperture 48 and into the freezerplate 20 through a circulation duct inlet 52. The direction of flow ofthe carbon dioxide from the inlet header 32 into the freezer plate 20 isindicated by the arrow 66.

[0033] Although FIG. 3 illustrates the inlet header 32 separated fromthe freezer plate 20, this is merely for illustrative purposes to showthe carbon dioxide flow from the inlet header 32 into the freezer plate20. In practice, the inlet and outlet headers 32, 34 will be securelyfastened to the freezer plates 20 by welding or other suitable means toprevent carbon dioxide leakage between the inlet header 32 and anindividual freezer plate 20.

[0034] The carbon dioxide enters the freezer plate 20 through thecirculation duct 50. The circulation duct 50 is shown in FIGS. 2, 3, 4and 8 to have an approximately elliptical cross-sectional shape. Thecross-sectional design of the circulation duct 50 of the presentinvention eliminates corners which are present in rectangular ducts andact as discrete regions of unacceptably high stress when carbon dioxideis used as a refrigerant. With the use of the cross-sectional shape ofthe present invention, the stress concentration factor is significantlyreduced from the level encountered with the use of prior circulationducts. The presence of elliptical ducts allows the present invention tosafely operate with carbon dioxide as a refrigerant. Additionally, theuse of nearly elliptical circulation ducts significantly reduces theamount of outward displacement experienced by the mid-point of thecirculation duct during the elevated internal pressures associated withthe use of carbon dioxide.

[0035] Upon entering the freezer plate 20, the carbon dioxide flowsthrough the serpentine circulation duct 50 as shown by the arrows 68 inFIG. 3. When the first end of the circulation duct 50, located towardsthe outlet end of the freezer plate, is encountered by the carbondioxide, the serpentine shape effects a 180 degree turn in the carbondioxide and directs it back towards the inlet header 32. When the carbondioxide reaches the inlet header 32 it then crosses over through another180 degree turn to the next elliptical duct, as shown by the arrows 70,to flow back towards the outlet header. This serpentine flow is repeatednumerous times, preferably seven with six 180 degree turns, althoughonly a single 180 degree turn is illustrated in FIG. 3.

[0036] Because the operating pressure of the carbon dioxide is higherthan that of ammonia (NH₃), the headers 32,34 must be sufficientlyrobust to withstand the increased operating pressure as well as theelevated pressures encountered when the refrigeration system is powereddown. For example, placing the web 62 between the first and secondchannels 58, 64 increases the structural integrity of the headers 32, 34so that the headers can safely handle the elevated pressures associatedwith the use of carbon dioxide as the refrigerant.

[0037] Although only one pass of the carbon dioxide through the freezerplate is illustrated in FIG. 3, a person of skill in the relevant artwould understand that multiple passes would be desirable to enhance theoverall freezer efficiency. In the preferred mode of operation, therefrigerant makes seven parallel passes with six 180 degree turnsthrough each freezer plate before exiting the freezer plate 20 andentering the outlet header. The seven preferred passes of therefrigerant through the elliptical serpentine duct 50 includes havingthe refrigerant flow substantially toward the outlet on four of thepasses and substantially towards the inlet on three of the passes thuscausing the refrigerant to exit the freezer plate on the opposite endfrom where it enters as shown in FIG. 2.

[0038] The repeated circulation of the carbon dioxide through theserpentine duct 50 allows the carbon dioxide to absorb heat that hasbeen transferred from the items located on the freezer plate 20 whichare to be frozen. Additionally, FIG. 3 illustrates the structuresassociated with the introduction of carbon dioxide into the freezerplates 20 through the inlet header 32. A substantially similar structureis also present (although not illustrated) on the outlet end of thefreezer plates 20.

[0039]FIG. 4 is a cross-sectional view of a freezer plate along line 4-4shown in FIG. 2. FIG. 4 illustrates the elliptical cross-section of thecirculation duct 50 located in each freezer plate 20. Although thecross-section shown in FIG. 4 illustrates only three elliptically shapedducts 50, any number of ducts, preferably seven, may be located with anindividual freezer plate 20.

[0040] The elliptical ducts 50 are formed in the freezer plate 20 whichis a solid but for the presence of the ducts 50 passing through thefreezer plate 20. The elliptical ducts each have a first diameter 72 anda second diameter 74. The ratio of the first diameter 72 to the seconddiameter 74 ranges from approximately 2.0 to approximately 2.35,preferably between about 2.1 and about 2.25 and most preferably about2.21.

[0041] As shown in FIG. 4, as well as FIGS. 1, 2, 3 and 6, the freezerplate 20 has a first generally planar heat transfer surface 80 forsupporting items to be frozen. The freezer plate 20 also has a secondgenerally planar heat transfer surface 82 spaced apart from the firstheat transfer surface 80 to define a solid volume therebetween.

[0042] The freezer plate 20 has a thickness 76 and a width 78. Thefreezer plate thickness 76 multiplied by the freezer plate width 78yields the total cross-sectional freezer plate area. Additionally, eachellipse shown as a cross section of the ducts 50 in FIG. 4 has an areaand the sum of the area for all ellipses present in the cross-section ofan individual freezer plate is the total ellipse area. The ratio of thetotal ellipse area to the total cross-sectional freezer plate arearanges from approximately 0.57 to approximately 0.67, preferably betweenabout 0.6 and about 0.64 and most preferably about 0.63.

[0043] Due to fundamental thermodynamic differences between ammonia(NH₃) and carbon dioxide (CO₂), including enthalpy and density, thepressure of the carbon dioxide in the freezer plate must besignificantly higher to achieve a similar freezing capacity relative toammonia. This increase in refrigerant pressure combined with priorfreezer plate designs placed the peak stress level of the freezer plateat or above the yield strength of aluminum 6061-T6, the material fromwhich freezer plates are preferably constructed. Additionally, thedisplacement experienced by prior freezer plate designs with theincrease in refrigerant pressures, was unacceptable. However, it was notdesirable to alter the material from which the freezer plates wereconstructed due to the favorable thermal conductivity, costs,manufacturing experience and industry acceptance of aluminum freezerplates.

[0044] By creating an elliptical duct 50 in an otherwise solid freezerplate as shown in FIG. 4, the maximum stress level in the freezer platewas dramatically reduced. The reduction in freezer plate stresssubstantially below the yield strength of aluminum 6061-T6 allowed theuse of carbon dioxide as a refrigerant while still allowing for anacceptable factor of safety.

[0045] Moreover, the use of an elliptical duct 50 has yielded anotheradvantage. In a plate freezer, the minimum operating temperature of thecarbon dioxide is −50° F (−46° C.). This temperature permits the use ofinexpensive carbon steel on some components as well as beingsufficiently far away from the triple point of carbon dioxide (−69° F.;−56° C.) at the working pressure. Because the operating temperature ofthe carbon dioxide (−50° F.; 46° C.) is lower than that for ammonia(−40° F.; −40° C.), the thermal efficiency of a freezer plate usingcarbon dioxide with elliptical ducts 50 is increased over the use ofammonia and prior freezer plate designs.

[0046]FIG. 5 is a cross-sectional view of a header along line 5-5 shownin FIG. 2. FIG. 5 illustrates the two-channeled header used inconjunction with the carbon dioxide refrigerant. Although shown as across-sectional view of the outlet header 34 in FIG. 2, the constructionof the inlet header 32 is identical.

[0047] As shown in FIG. 2 and FIG. 5, the carbon dioxide exits a freezerplate 20 through a circulation duct outlet 56 and into the secondchannel 64 through the outlet header aperture 54. The first channel 64and the circulation duct outlet 56 are in fluid communication throughthe outlet header aperture 54 to allow the carbon dioxide to flow fromthe freezer plate 20 into the outlet header 34. Apertures (identified inFIG. 3 with reference numeral 60) located in the web 62 which separatesthe first channel 64 from the second channel 58, allow the carbondioxide to flow from the first channel 64 into the second channel 58.The carbon dioxide then exits the second channel 58 through an outlethose 36 into the collecting manifold 38 as shown in FIGS. 1 and 8.

[0048] With the use of carbon dioxide as a refrigerant and theconcomitant increase in refrigerant pressure combined with inadequateprior inlet and outlet header designs, the peak stress levels in theheader were above the yield strength of aluminum 6061-T6 from which theheaders are preferably constructed.

[0049] However, by designing a more robust header for use with thecarbon dioxide refrigerant, including the addition of the web 62, themaximum stress level in each header was dramatically reduced. Thisreduction in maximum header stress, to a level substantially below theyield strength of aluminum, allowed the use of carbon dioxide as therefrigerant while still maintaining an acceptable factor of safety andminimizing the amount of material required for constructing the headers.

[0050]FIG. 6 is a cross-sectional view of a freezer plate along line 4-4in FIG. 2 showing the magnified displacement of a freezer plate when theinternal pressure in the elliptical ducts is approximately 1400 psig(9660 kPa).

[0051]FIG. 6 is an illustration from a finite element analysis showingthe manner in which the elliptical ducts 50 and the first and secondheat transfer surfaces 80, 82 of the freezer plate 20 deflect when theinternal pressure in the elliptical ducts is approximately 1400 psig(9660 kPa). Based upon this finite element analysis, the areas of thefreezer plate cross section with the highest stress and maximumdeflection are identified with reference numeral 84 in FIG. 6. Theelliptical ducts 50 of the present invention has reduced the magnitudeof maximum stress and deflection 84 experienced by the freezer plate at1400 psig (9660 kPa) by a factor of approximately three.

[0052] The use of the elliptical ducts has placed the maximum stress inthe freezer plate 20 at a level substantially below the yield strengthof the material from which the freezer plate 20 is typicallyconstructed. With the significant reduction in the maximum stress due tothe use of the elliptical ducts 50, the carbon dioxide could be used asa refrigerant without replacing the aluminum 6061-T6. More importantly,using elliptical ducts to reduce the maximum stress to a levelsubstantially below the yield strength, a factor of safety is nowdesigned into the freezer plate 20.

[0053]FIG. 7 is a cross-sectional view of an outlet header 34 showingthe magnified displacement of a freezer plate when the internal pressurewithin the header is approximately 1400 psig (9660 kPa). Althoughdescribed in terms of the outlet header 34, the description whichfollows is equally applicable to the inlet headers 32.

[0054]FIG. 7 is an illustration from a finite element analysis showingthe manner in which the first channel 58 and the second channel 64deflect when the internal pressure in the channels is approximately 1400psig (9660 kPa). Based upon this finite element analysis, the areas ofthe header cross section with the highest stress and maximum deflectionare identified with reference numeral 86 in FIG. 7. The presence of theweb 62 in the header 34 has reduced the region of maximum stress anddeflection 86 experienced by the freezer plate when subjected to arefrigerant pressure of approximately 1400 psig (9660 kPa) by a factorof approximately five.

[0055] The use of the two-channel header has placed the maximum stressin the header 34 at a level substantially below the yield strength ofthe material from which the header 20 is typically constructed. With thesignificant reduction in the maximum stress due to the use of thetwo-channel header, the carbon dioxide could be used as a refrigerantwithout replacing the aluminum 6061-T6. More importantly, using atwo-channel header to reduce the maximum stress to a level substantiallybelow the yield strength, a factor of safety is now designed into theheader 34.

[0056]FIG. 8 is a diagrammatic illustration of the mechanicalrefrigeration system 10 for use in conjunction with the presentinvention. In operation, a pump 22 draws liquid carbon dioxide from aprimary receiver 24 in the direction shown by flow arrows 26 a. Liquidcarbon dioxide is then discharged from the pump 22 (shown by flow arrows26 b) into the liquid manifold 28. The cold liquid carbon dioxide passesfrom the liquid manifold 28 into the freezer plates 20 through one ofseveral inlet hoses 30 which connect the liquid manifold 28 with theindividual inlet headers 32. The liquid carbon dioxide refrigerantpasses through an inlet header 32 and into one of the freezer plates 20.

[0057] The freezer plates 20 are located in the freezer space and theitems to be frozer (not illustrated herein) are placed on top of afreezer plate 20 to allow freezing to occur. A plurality of individualfreezer plates 20 are placed in side-by-side orientation as shown inFIGS. 1 and 2. The freezer plates 20 are then fastened together, usuallyby welding to produce a freezer plate shelf for supporting the items tobe frozen. The individual freezer plates 20 are generally solid aluminum6061-T6 except for the elliptical duct formed into each freezer platefor circulating the refrigerant. Heat transfer from the items to befrozen through the freezer plates 20 and into the carbon dioxide causessome of the liquid carbon dioxide to evaporate. This evaporationproduces a saturated mixture of liquid and vapor carbon dioxide whichexits the freezer plates 20 through the outlet headers 34. The saturatedmixture of liquid and vapor carbon dioxide then exits the outlet headers34 through outlet hoses 36 and proceeds into the collecting manifold 38.

[0058] The liquid/vapor carbon dioxide exits the collecting manifold 38as shown by flow arrow 26 c and returns to the primary receiver 24 torepeat the cycle with the liquid portion of liquid/vapor carbon dioxidewhich has been returned to the primary receiver 24.

[0059] The continuous conversion of a portion of the liquid carbondioxide present in the primary receiver 24 to gaseous carbon dioxidewould eventually convert all of the liquid carbon dioxide to gaseouscarbon dioxide and prevent the continued cooling of the freezer plates20. To replenish the liquid carbon dioxide in the primary receiver 24,the gaseous carbon dioxide located in the primary receiver 24 is pumpedfrom the primary receiver 24 by a compressor 40 in the direction shownby flow arrows 42 a in FIG. 8. The super-heated vapor drawn from theprimary receiver is then compressed by the compressor 40 causing anincrease in pressure and temperature of the carbon dioxide. The hot,high-pressure carbon dioxide is then circulated through a condenser 44in the direction shown by flow arrow 42 b. The carbon dioxide is thencooled through heat transfer to the environment in the condenser 44 toform liquid carbon dioxide which flows from the condenser 44 in thedirection shown by flow arrow 42 c. The resulting liquid carbon dioxideis then collected in an intermediate receiver 46. The liquid carbondioxide is then circulated from the intermediate receiver 46 to theprimary receiver 24 in the direction shown by flow arrow 42 d, thereby,replenishing the liquid carbon dioxide in the primary receiver 24.

[0060] Accordingly, an evaporator for use in a vapor-compressionrefrigeration cycle meeting the aforestated objectives has beendescribed. It should be understood, however, that the foregoingdescription has been limited to the presently contemplated best mode forpracticing the invention in a specific application using carbon dioxideas a refrigerant. It will be apparent to one of skill in the relevantart that various modifications may be made to the invention, with theattainment of some or all of the advantages of the invention.Accordingly, the invention should only be limited by the appended claimsand equivalents thereof, which claims are intended to cover such othervariations and modifications as come within the spirit and scope of theinvention.

What is claimed is:
 1. An evaporator adapted for use in avapor-compression refrigeration cycle in a plate freezer comprising: alongitudinally extending plate body having a first generally planar heattransfer surface, a second generally planar heat transfer surface spacedapart from the first heat transfer surface, to define a plate body solidvolume; and at least one longitudinally extending duct passing throughthe plate body solid volume to channel a refrigerant maintained at arelatively high pressure, the duct having an elliptical cross-sectionwhich maintains a stress level in the plate body, caused by therelatively high pressure refrigerant, at a level substantially below theyield strength of the material from which the plate body is constructed.2. The invention as in claim 1 wherein the spacing between the first andsecond heat transfer surfaces and the dimensions of the elliptical ductare such that the von Mises stress is less than the yield strength ofthe material from which the evaporator is constructed when therefrigerant has a pressure of approximately 1400 psig.
 3. The inventionas in claim 1 wherein at least one heat transfer surface contacts itemsto be frozen in a plate freezer.
 4. The invention as in claim 1 whereinboth heat transfer surfaces contact items to be frozen in a platefreezer.
 5. The invention as in claim 1 wherein the duct extendsthroughout substantially the entire plate body in a serpentine manner.6. The invention as in claim 5 wherein the plate body has a length and awidth with the length substantially greater than the width and theserpentine duct extends substantially throughout the entire plate bodyalong the length of the plate body.
 7. The invention as in claim 5wherein the serpentine duct makes seven passes through the plate body.8. The invention as in claim 1 wherein the ratio between the totalellipse area to the total cross-sectional freezer-plate area is betweenabout 0.57 and about 0.67.
 9. The invention as in claim 1 wherein eachelliptical duct has a first diameter and a second diameter, wherein theratio between the first diameter and second diameter is between about2.0 and about 2.35.
 10. The invention as in claim 1 wherein therefrigerant passing through the evaporator is a CFC refrigerant.
 11. Theinvention as in claim 1 wherein the refrigerant passing through theevaporator is a non-CFC refrigerant.
 12. The invention as in claim 1wherein the refrigerant passing through the evaporator is carbondioxide.
 13. The invention as in claim 1 wherein the refrigerant passingthrough the evaporator is ammonia.
 14. The invention as in claim 1wherein the refrigerant passing through the evaporator is at a pressurebetween about 100 psig and about 300 psig.
 15. The invention as in claim14 wherein the refrigerant passing through the evaporator is carbondioxide.
 16. A plate freezer comprising: a compartment wherein thetemperature of the compartment is less than or equal to approximately 0°Celsius; and a plurality of spaced-apart shelves located in thecompartment with each of the shelves adapted to receive items to befrozen between the adjacent shelves, each of the shelves include aplurality of generally rectangular plates having a length and a widthwith the length substantially greater than the width, the plates aredisposed in an abutting relationship along their respective lengths,each plate has a first generally planar heat transfer surface, a secondgenerally planar heat transfer surface spaced apart from the first heattransfer surface, to define a plate body solid volume; and at least onelongitudinally extending duct passing through the plate body solidvolume to channel a refrigerant maintained at a relatively highpressure, the duct having an elliptical cross-section which maintains astress level in the plate body, caused by the relatively high pressurerefrigerant, at a level substantially below the yield strength of thematerial from which the plate body is constructed.
 17. The invention asin claim 16 wherein the spacing between the first and second surfacesand the dimensions of the elliptical duct are such that the von Misesstress is less than the yield strength of the material from which theplate is constructed when the refrigerant has a pressure ofapproximately 1400 psig.
 18. The invention as in claim 16 wherein theduct extends throughout substantially the entire plate body in aserpentine manner.
 19. The invention as in claim 18 wherein theserpentine duct extends substantially throughout the entire plate bodyalong the length of the plate body.
 20. The invention as in claim 18wherein the serpentine duct makes seven passes through the plate body.21. The invention as in claim 16 wherein the ratio between the totalellipse area to the total cross-sectional freezer-plate area is betweenabout 0.57 and about 0.67.
 22. The invention as in claim 16 wherein eachelliptical duct has a first diameter and a second diameter, wherein theratio between the first diameter and second diameter is between about2.0 and about 2.35.
 23. The invention as in claim 16 wherein therefrigerant passing through the plate is a CFC refrigerant.
 24. Theinvention as in claim 16 wherein the refrigerant passing through theplate is a non-CFC refrigerant.
 25. The invention as in claim 16 whereinthe refrigerant passing through the plate is carbon dioxide.
 26. Theinvention as in claim 16 wherein the refrigerant passing through theplate is ammonia.
 27. The invention as in claim 16 wherein therefrigerant passing through the plate is at a pressure between about 100psig and about 200 psig.
 28. The invention as in claim 27 wherein therefrigerant passing through the evaporator is carbon dioxide.
 29. Anevaporator for a plate freezer comprising: a plurality of spaced-apartshelves located in the compartment with each of the shelves adapted toreceive items to be frozen between the adjacent shelves, each of theshelves include a plurality of generally rectangular plates having alength and a width with the length substantially greater than the width,the plates are disposed in an abutting relationship along theirrespective lengths, each plate has a first generally planar heattransfer surface, a second generally planar heat transfer surface spacedapart from the first heat transfer surface, to define a plate body solidvolume; at least one longitudinally extending duct passing through theplate body solid volume to channel a refrigerant maintained at arelatively high pressure, the duct having an elliptical cross-sectionwhich maintains a stress level in the plate body, caused by therelatively high pressure refrigerant, at a level substantially below theyield strength of the material from which the plate body is constructed;and wherein the ratio between the total ellipse area in a plate to thetotal cross-sectional freezer plate area of that plate is between about0.57 and about 0.67.
 30. The invention as in claim 29 wherein thespacing between the first and second heat transfer surfaces and thedimensions of the elliptical duct are such that the von Mises stress isless than the yield strength of the material from which the evaporatoris constructed when the refrigerant has a pressure of approximately 1400psig.
 31. The invention as in claim 29 wherein the duct extendsthroughout substantially the entire plate body in a serpentine manner.32. The invention as in claim 31 wherein the plate body has a length anda width with the length substantially greater than the width and theserpentine duct extends substantially throughout the entire plate bodyalong the length of the plate body.
 33. The invention as in claim 31wherein the serpentine duct makes seven passes through the plate body.34. The invention as in claim 29 wherein each elliptical duct has afirst diameter and a second diameter, wherein the ratio between thefirst diameter and second diameter is between about 2.0 and about 2.35.35. The invention as in claim 29 wherein the refrigerant passing throughthe evaporator is a CFC refrigerant.
 36. The invention as in claim 29wherein the refrigerant passing through the evaporator is a non-CFCrefrigerant.
 37. The invention as in claim 29 wherein the refrigerantpassing through the evaporator is carbon dioxide.
 38. The invention asin claim 29 wherein the refrigerant passing through the evaporator isammonia.
 39. The invention as in claim 29 wherein the refrigerantpassing through the evaporator is at a pressure between about 100 psigand about 200 psig.
 40. The invention as in claim 39 wherein therefrigerant passing through the evaporator is carbon dioxide.